As is well known, in an optical modulator, a traveling-wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) in which an optical waveguide and a traveling-wave electrode are formed in a substrate made of a material such as lithium niobate (LiNbO3) is applied to large-capacity transmission systems of 2.5 Gbit/s and 10 Gbit/s because of its excellent chirping characteristics. The lithium niobate has the so-called electro-optic effect in which the refractive index is changed by applying an electric field (hereinafter the lithium niobate substrate is abbreviated as LN substrate).
Recently a study of the application of the LN optical modulator to a very-large-capacity transmission system of 40 Gbit/s has been conducted, and the LN optical modulator is expected to be a key device in the large-capacity transmission system.
FIG. 12 is a perspective view showing a configuration of an LN optical modulator 100 according to a first prior art disclosed in Patent Reference 1.
Referring to FIG. 12, the reference numeral 1 designates a z-cut LN substrate, the reference numeral 2 designates a Mach-Zehnder type optical waveguide which is formed by thermal diffusion of Ti, the reference numeral 2a designates an input optical waveguide, the reference numeral 2b designates a Y-branching type of branching optical waveguide, the reference numerals 2c-1 and 2c-2 designate an interaction optical waveguide, the reference numeral 2d designates a Y-branching type of multiplexing optical waveguide, the reference numeral 2e designates an output optical waveguide, and the reference numeral 2g designates an edge portion of the output optical waveguide.
Further, in FIG. 12, the reference numeral 3 designates an electric signal source, the reference numeral 4 designates a center electrode of the traveling-wave electrode, the reference numerals 5a and 5b designate a ground electrode, the reference numerals 6a and 6b designate a radiant light beam which is generated when an optical signal is in an off state as described later, the reference numeral 7 designates a single-mode optical fiber for a signal light beam, the reference numeral 8 designates an optical fiber for receiving a radiant light beam, the reference numeral 11 designates a monitor photodetector which is formed by, e.g., a photodiode, the reference numeral 9 designates a radiant light beam detection means including a bias controller which adjusts an operating point of a bias power supply DC and an operating point of an LN optical modulator 100 based on a radiant light detection signal as described later from the monitor photodetector 11.
FIGS. 13A, 13B, and 13C are a view for explaining the operational principle of the LN optical modulator 100 which is configured as shown in FIG. 12.
FIGS. 13A and 13B are a view for explaining the operation of an optical waveguide 2, and FIG. 13C is a side view showing the LN optical modulator 100.
The operation of the LN optical modulator 100 will be described with reference to FIG. 12 and FIGS. 13A, 13B, and 13C.
First a light beam incident on the input optical waveguide 2a is divided into two beams in the branching optical waveguide 2b. 
When the electric signal from the electric signal source 3 is not applied between the center electrode 4 of the traveling-wave electrode and the ground electrodes 5a and 5b, as shown in FIG. 13A, the light beams propagate through the interaction optical waveguides 2c-1 and 2c-2 with the in-phase each other.
Then, the light beams are multiplexed by the multiplexing optical waveguide 2d, and the light beam propagates through the output optical waveguide 2e as a basic mode. Finally the light beam is output to the single-mode optical fiber 7 for the signal light beam.
This is referred to as the on state. A point where the multiplexing optical waveguide 2d is coupled to the output optical waveguide 2f is referred to as multiplexing point 2h. 
On the other hand, when the electric signal from the electric signal source 3 is applied between the center electrode 4 of the traveling-wave electrode and the ground electrodes 5a and 5b, as shown in FIG. 13B, the light beams propagate through the interaction optical waveguides 2c-1 and 2c-2 with the anti-phase each other.
Then, the light beams are multiplexed by the multiplexing optical waveguide 2d to form a high-order mode light beam of the first-order.
Usually the output optical waveguide 2e is designed to cut off the high-order mode light beam of the first-order.
Therefore, since the high-order mode light beam of the first-order cannot propagate through the output optical waveguide 2e, the high-order mode light beam of the first-order is radiated in the substrate 1 as radiant light beams 6a and 6b with a small angle of 0.7 degrees with respect to a horizontal direction as shown in FIG. 13B, the high-order mode light beam of the first-order is radiated in the substrate 1 with a small angle of 0.9 degrees with respect to a depth direction as shown in FIG. 13C, and the high-order mode light beam of the first-order propagates through the substrate 1 while broadened. This is referred to as the off state.
In a voltage-optical output characteristic shown in FIG. 14, a curve indicated by a solid line shows a certain state of the voltage-optical output characteristic of the LN optical modulator 100, and Vb designates a DC bias voltage at that state.
As shown in FIG. 14, usually the DC bias voltage Vb is set at a midpoint between a peak and a bottom of the optical output characteristic curve.
On the other hand, as shown in FIG. 14 by a broken line, when the voltage-optical output characteristic is changed for some reason such as temperature change, it is necessary that the setting of the bias point is changed to Vb′.
In this first prior art, the radiant light beam is received by the optical fiber 8 for receiving the radiant light beam and the radiant light beam propagates through the optical fiber 8 for receiving the radiant light beam, and then the radiant light beam is converted into current by causing the radiant light beam to be incident or the monitor photodetector 11 made of a photodiode, for example.
The radiant light detection means 9 including the bias controller detects the change in voltage-optical output characteristic by magnitude of the current, and the radiant light detection means 9 finds the optimum bias point of the DC bias voltage by the bias power supply DC.                Patent Reference 1: Jpn. Pat. Appln. KOKAI Publication No. 3-145623        
However, in the LN optical modulator 100 having the above configuration, there are the following problems.
Actually, as shown in FIGS. 13B and 13C, since the radiant light beams are output downward into the substrate 1 with the small angle of 0.7 degrees with respect to the horizontal direction of the substrate 1 and the small angle of 0.9 degrees with respect to the depth direction, it is necessary that the optical fiber 8 for receiving the radiant light beam is arranged very close to the single-mode optical fiber 7 for the signal light beam and at a position which is slightly lower than the single-mode optical fiber 7 for the signal light beam.
FIG. 15 shows an optical-signal off state when viewed from the single-mode optical fiber 7 for the signal light beam side.
In FIG. 13B, for example, assuming that a length in an optical axis direction of the output optical waveguide 2e is 4 mm, since the propagation angle is only 0.7 degrees in the horizontal direction of the radiant light beam as described above, a space between the single-mode optical fiber 7 for the signal light beam and the radiant light beam 6a or the radiant light beam 6b is as extremely narrow as about 50 μm. Therefore, it is very difficult that the single-mode optical fiber 7 for the signal light beam and the optical fiber 8 for receiving the radiant light beam are mounted together.
It will be described with reference to FIGS. 16A and 16B (for example, see FIG. 9 in Patent Reference 1).
Referring to FIGS. 16A and 16B, the reference numeral 7a designates a core of the single-mode optical fiber for the signal light beam, the reference numeral 8a designates a core of the radiant light reception optical fiber, and the reference numeral 10a designates a capillary (the capillary is made of a dielectric material, usually a glass material is used, however, other material such as a ceramic material may be used).
A hole which is different from a hole for the single-mode optical fiber 7 for the signal light beam is made in the capillary 10, and the optical fiber 8 for receiving the radiant light beam is fixed into the hole.
Thus, each positional relationship is adjusted and fixed such that the signal light beam is coupled to the core 7a of the single-mode optical fiber 7 for the signal light beam and such that the radiant light beam 6b (or 6a) is coupled to the core 8a of the optical fiber 8 for receiving the radiant light beam.
As described above, in the LN optical modulator 100 according to the first prior art, in spite of the distance between the signal light beam and the radiant light beam is as extremely small as about 50 μm, the mounting, in which the signal light beam is coupled to the core 7a of the single-mode optical fiber 7 for the signal light beam and the radiant light beam is coupled to the core 8a of the optical fiber 8 for receiving the radiant light beam, is required. Therefore, since the mounting is very difficult to perform, the development of the LN optical modulator having the structure in which the mounting is easy to perform is demanded.
Usually, in order to avoid the difficulty of the mounting, it is thought that the distance between the signal light beam and the radiant light beam is broadened.
Aside from the idea that the distance between the signal light beam and the radiant light is broadened, there is disclosed a technology in which an interference pattern is formed far away from the signal light beam by causing the radiant light beam to interfere with the signal light beam (for example, see Patent Reference 2).
However, in the technology, the interference between the radiant light beam and the signal light beam means that the signal light beam is attenuated. As a result, there is the problem that the interference leads to an increase in loss of the signal light beam or the interference pattern can be formed only in a range on which the signal light beam has an influence, i.e., in an area relatively close to the signal light beam because of the mere interference. Therefore, actually the technology disclosed in Patent reference 2 cannot solve the problem of the mounting difficulty.                Patent Reference 2: Jpn. Pat. Appln. KOKAI Publication No. 10-228006        
Thus, it is very difficult that both the single-mode optical fiber 7 for the signal light beam and the optical fiber 8 for receiving the radiant light beam are mounted in the capillary 10a. 
Therefore, instead of the use of the optical fiber 8 for receiving the radiant light beam, it is thought that the radiant light beam is received by the monitor photodetector 11 such as the monitor photodiode after the radiant light beam passes through the capillary 10a. 
In this case, assuming that refractive indexes of the z-cut LN substrate 1 and the capillary 10a are set at 2.14 and 1.45 respectively, because the radiant light beam propagates through the capillary 10a with an angle of refraction of ±0.7°×2.14/1.45=±1.0°, the radiant light beam propagates extremely close to the single-mode optical fiber 7 for the signal light beam fixed in the capillary 10a. Therefore, actually it is difficult to mount the monitor photodetector 11 such as the monitor photodiode.
FIG. 17 shows the LN optical modulator 100 according to a second prior art which is of the structure for solving these problems.
In the LN optical modulator 100 according to the second prior art, the radiant light beams 6a and 6b propagating through the z-cut LN substrate 1 are caused to propagate as radiant light beams 6c and 6d through a capillary 10b whose rear end is inclined.
At this point, total reflection of the light beam is performed to output the light beam to the outside by previously depositing a dielectric multi-layer film 14 on the rear-end inclined surface of the capillary 10b, and the light beam is received by the monitor photodetector 11 such as the monitor photodiode to convert the light beam into the current.
However, in the LN optical modulator 100 according to the second prior art, there are the following serious problems. Then, the problems will be discussed.
First an optical path length, in which the radiant light beams 6a and 6b reach to the monitor photodetector 11 such as the monitor photodiode after passing through the substrate facet 1a, will be considered.
In the case of the radiant light beam 6c, the radiant light beam 6c propagates through the capillary 10b by a distance of L1 after passing through the substrate facet 1a. Then, the radiant light beam 6c is reflected at the dielectric multi-layer film 14 on the rear-end inclined surface of the capillary 10b, and the radiant light beam 6c propagates upward through the capillary 10b by a distance of L2.
Then, the radiant light beam 6c propagates through air by a distance of L3, and the radiant light beam 6c reaches the monitor photodetector 11 such as the monitor photodiode.
Assuming that the refractive index of the capillary 10d is nc, a total optical path length L6c through which the radiant light beam 6c propagates becomes L6c=ncL1+ncL2+L3.
On the other hand, in the case of the radiant light beam 6d, the radiant light beam 6d propagates through the capillary 10b by a distance of L4 after passing through the substrate facet 1a. Then, the radiant light beam 6d is reflected at the dielectric multi-layer film 14 on the rear-end inclined surface of the capillary 10b, and the radiant light beam 6d propagates upward through the capillary 10b by a distance of L5.
Then, the radiant light beam 6d propagates through air by a distance of L6, and the radiant light beam 6d reaches the monitor photodetector 11 such as the monitor photodiode.
A total optical path length L6d through which the radiant light beam 6d propagates becomes L6d=ncL4+ncL5+L6.
On the other hand, the radiant light beams 6a and 6b propagate through the capillary 10b with different angles of ±1.0°, and the radiant light beams 6a and 6b are reflected upward at the dielectric multi-layer film 14 on the rear-end inclined surface of the capillary 10b. Then, as shown in FIG. 18, the radiant light beams 6c and 6d overlap each other to generate the interference when the radiant light beams 6c and 6d are incident on the monitor photodetector 11 such as the monitor photodiode.
FIG. 19 shows a state in which phases of the radiant light beams 6c and 6d differ from each other by about 180 degrees.
Thus, when the phases of the radiant light beams 6c and 6d differ from each other by 180 degrees, as shown in FIG. 20A, there is a point where power of the radiant light beams 6c and 6d become zero in the overlapped portion.
However, because the refractive index nc of the capillary 10b varies depending on temperature, the optical path lengths L6c and L6d vary depending on the temperature when the radiant light beams 6c and 6d are incident on the monitor photodetector 11 such as the monitor photodiode.
As a result, since a phase difference between the radiant light beams 6c and 6d is different from 180 degrees, the power of the overlapped portion between the radiant light beams 6c and 6d never becomes zero at any point as shown in FIG. 20B.
In other words, as shown in FIG. 18 to FIG. 20B, the light intensity of the overlapped portion between the radiant light beams 6c and 6d varies depending on the temperature, so that a trouble occurs in the DC bias control of the LN optical modulator 100.
Further, as with the capillary 10a in the first prior art shown in FIG. 16, in the second prior art, it is also necessary that the single-mode optical fiber 7 for the signal light beam is mounted in the capillary 10b. 
In both the capillaries 10a and 10b, in order to facilitate the mounting of the single-mode optical fiber 7 for the signal light beam, it is desirable that a guide spot facing larger than an outer shape of the single-mode optical fiber 7 for the signal light beam is made in the rear ends of the capillaries 10a and 10b. 
However, since the radiant light beam propagates near the single-mode optical fiber 7 for the signal light beam in both the capillaries 10a and 10b, the guide spot facing cannot be provided in the rear ends of the capillaries 10a and 10b. 
FIG. 21 shows the LN optical modulator 100 according to a third prior art which is of the structure for solving these problems.
In the LN optical modulator 100 according to the third prior art, because only the radiant light beam 6c is received by the monitor photodetector 11 such as the monitor photodiode, as shown in FIG. 21, it is devised that a capillary 10c is formed in the shape in which a half of the capillary 10c is cut off such that the radiant light beam 6d is not incident on the monitor photodetector 11 such as the monitor photodiode.
The half cut-off shape of the capillary 10c enables the guidance of the mounting of the single-mode optical fiber 7 for the signal light beam.
However, in the structure of the LN optical modulator 100 according to the third prior art, it is necessary that the capillary 10c is machined in the complicated structure. Further, as with the capillary 10b of the LN optical modulator 100 according to the second prior art shown in FIG. 17, it is necessary that a dielectric multi-layer film 15 for performing the total reflection of the light is deposited on the rear-end inclined surface of the capillary 10c. Therefore, there is the problem that production cost of the optical modulator is further increased as a whole.
In the structure of the LN optical modulator 100 according to the third prior art, since the light which can be received by the monitor photodetector 11 such as the monitor photodiode is only the radiant light beam 6c, there is also the problem the power becomes a half.                Patent Reference 3: Japanese Patent Application No. 2000-101316        